From 2b6958788276f99a2217d967de947f2e8dff78af Mon Sep 17 00:00:00 2001
From: mberk-yilmaz <mustafa.yilmaz@redearge.com>
Date: Wed, 18 Mar 2026 12:48:20 +0300
Subject: [PATCH 1/2] addon: add const overloads for multipart_t front/back

Add const-qualified overloads for multipart_t::front() and back() in `zmq_addon.hpp`.

Note on API shape:
- In a container-style API, non-const overloads would typically return `message_t&`.
- This change intentionally preserves existing non-const return types (`const message_t&`)
  for backward-compatibility and to avoid broadening mutability semantics in this PR.

Also extend `tests/multipart.cpp` with compile-time and runtime checks for const/non-const access paths.

No behavioral change intended.
---
 tests/multipart.cpp | 29 +++++++++++++++++++++++++++--
 zmq_addon.hpp       |  6 ++++++
 2 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/tests/multipart.cpp b/tests/multipart.cpp
index 6901517..4af4948 100644
--- a/tests/multipart.cpp
+++ b/tests/multipart.cpp
@@ -1,6 +1,8 @@
 #include <catch2/catch_all.hpp>
 #include <zmq_addon.hpp>
 
+#include <utility>
+
 #ifdef ZMQ_HAS_RVALUE_REFS
 
 #ifdef ZMQ_CPP17
@@ -17,6 +19,22 @@ static_assert(std::is_invocable<decltype(&zmq::multipart_t::recv),
 #endif
 static_assert(std::is_constructible<zmq::multipart_t, zmq::socket_ref>::value,
               "Can't construct with socket_ref");
+static_assert(
+  std::is_same<decltype(std::declval<zmq::multipart_t &>().front()),
+               const zmq::message_t &>::value,
+  "multipart_t::front() should keep returning const message_t&");
+static_assert(
+  std::is_same<decltype(std::declval<const zmq::multipart_t &>().front()),
+               const zmq::message_t &>::value,
+  "multipart_t::front() const should return const message_t&");
+static_assert(
+  std::is_same<decltype(std::declval<zmq::multipart_t &>().back()),
+               const zmq::message_t &>::value,
+  "multipart_t::back() should keep returning const message_t&");
+static_assert(
+  std::is_same<decltype(std::declval<const zmq::multipart_t &>().back()),
+               const zmq::message_t &>::value,
+  "multipart_t::back() const should return const message_t&");
 
 /// \todo split this up into separate test cases
 ///
@@ -106,11 +124,18 @@ TEST_CASE("multipart legacy test", "[multipart]")
     multipart.pushmem("Frame0", 6);
     assert(multipart.size() == 10);
 
-    const message_t &front_msg = multipart.front();
+    const message_t &front_msg_nonconst = multipart.front();
+    assert(&front_msg_nonconst == &multipart[0]);
+
+    const multipart_t &const_multipart = multipart;
+    const message_t &front_msg = const_multipart.front();
     assert(multipart.size() == 10);
     assert(std::string(front_msg.data<char>(), front_msg.size()) == "Frame0");
 
-    const message_t &back_msg = multipart.back();
+    const message_t &back_msg_nonconst = multipart.back();
+    assert(&back_msg_nonconst == &multipart[multipart.size() - 1]);
+
+    const message_t &back_msg = const_multipart.back();
     assert(multipart.size() == 10);
     assert(std::string(back_msg.data<char>(), back_msg.size()) == "Frame9");
 
diff --git a/zmq_addon.hpp b/zmq_addon.hpp
index 168735d..04c9966 100644
--- a/zmq_addon.hpp
+++ b/zmq_addon.hpp
@@ -594,9 +594,15 @@ class multipart_t
     // get message part from front
     const message_t &front() { return m_parts.front(); }
 
+    // get message part from front
+    const message_t &front() const { return m_parts.front(); }
+
     // get message part from back
     const message_t &back() { return m_parts.back(); }
 
+    // get message part from back
+    const message_t &back() const { return m_parts.back(); }
+
     // Get pointer to a specific message part
     const message_t *peek(size_t index) const { return &m_parts[index]; }
 

From 1d691c21a73f516580a6c59039f78b2451aada9b Mon Sep 17 00:00:00 2001
From: mberk-yilmaz <mustafa.yilmaz@redearge.com>
Date: Wed, 18 Mar 2026 14:31:29 +0300
Subject: [PATCH 2/2] fix: applied clang-format to fix style. No functional
 changes.

---
 tests/multipart.cpp | 21 +++++++++------------
 1 file changed, 9 insertions(+), 12 deletions(-)

diff --git a/tests/multipart.cpp b/tests/multipart.cpp
index 4af4948..1004410 100644
--- a/tests/multipart.cpp
+++ b/tests/multipart.cpp
@@ -19,22 +19,19 @@ static_assert(std::is_invocable<decltype(&zmq::multipart_t::recv),
 #endif
 static_assert(std::is_constructible<zmq::multipart_t, zmq::socket_ref>::value,
               "Can't construct with socket_ref");
-static_assert(
-  std::is_same<decltype(std::declval<zmq::multipart_t &>().front()),
-               const zmq::message_t &>::value,
-  "multipart_t::front() should keep returning const message_t&");
+static_assert(std::is_same<decltype(std::declval<zmq::multipart_t &>().front()),
+                           const zmq::message_t &>::value,
+              "multipart_t::front() should keep returning const message_t&");
 static_assert(
   std::is_same<decltype(std::declval<const zmq::multipart_t &>().front()),
                const zmq::message_t &>::value,
   "multipart_t::front() const should return const message_t&");
-static_assert(
-  std::is_same<decltype(std::declval<zmq::multipart_t &>().back()),
-               const zmq::message_t &>::value,
-  "multipart_t::back() should keep returning const message_t&");
-static_assert(
-  std::is_same<decltype(std::declval<const zmq::multipart_t &>().back()),
-               const zmq::message_t &>::value,
-  "multipart_t::back() const should return const message_t&");
+static_assert(std::is_same<decltype(std::declval<zmq::multipart_t &>().back()),
+                           const zmq::message_t &>::value,
+              "multipart_t::back() should keep returning const message_t&");
+static_assert(std::is_same<decltype(std::declval<const zmq::multipart_t &>().back()),
+                           const zmq::message_t &>::value,
+              "multipart_t::back() const should return const message_t&");
 
 /// \todo split this up into separate test cases
 ///